Phenylketonuria (PKU) is a metabolic disorder which causes mental retardation when untreated. According to evidence generated by the Collaborative Study of Children Treated for Phenylketonuria (PKUCS), such damage can be prevented by the initiation of restricted pheylalanine (phe) dietary therapy during the first month of life and carefully monitored during subsequent years. Two hundred and sixteen children have been studied since the inception of the PKCUS on October 1, 1967 and they now range in age from 7 to 12 years. Phase A followed each child through age 6 years. In the second Phase (Phase B), children who were treated for the first 6 years of life were randomized to either remain on the diet or terminate therapy at 6 years. Phase B will terminate when all subjects have reached age 8. The proposed project constitutes the third phase of the PKUCS and would compare children remaining on the diet after 10 years with those who discontinued the diet after 6 years. This study, to be conducted over a 4 year period would attempt to answer 4 questions. Is there a difference at age 12 years on IQ or school achievement between PKU children on a phe-restricted diet (DC1) and PKU children who discontinued the phe-restricted diet prior to age 8 years (DD2)? Is there a difference at age 12 on physical, neurological or growth parameters between DC1 children and their non-PKU siblings? Is there a difference at age 12 on physical, neurological or growth parameters between DD2 children and their non-PKU siblings? Is there a difference at age 12 on measures of self-esteem and behavior between DC1 and DD2 children?